It is usual for the primary circuit to be isolated from ground both at the continuous operation voltage and at test voltages as required by regulations, while the secondary circuit is isolated from the primary circuit and generally has one of its terminals grounded. The magnetic circuit is either grounded or connected to the high-tension potential depending on whether the primary/secondary insulation is provided for the primary winding or for the secondary winding(s).
In the case of a grounded magnetic circuit, said circuit and the secondary circuit are sunk in a solid insulating material or are inserted in a casing which is itself grounded and filled with a liquid or gaseous insulating fluid. These casings are voluminous so as to contain the magnetic circuits and the outputs of the terminals of the secondary windings require sealed lead-through bushings which are difficult and expensive to produce. The magnetic circuits are inaccessible.
As for the solid insulation used between the live parts and ground, it is subjected to great dielectric stress and may be subject to ageing. In contrast, current transformers for high-tension installations generally have a high nominal current and hence the primary circuit comprises a single turn. In these conditions, even during a short circuit or a lightning strike, the voltage across the terminals of the primary circuit always remains very low.
It is therefore advantageous to avoid having organic insulating material between the primary conductor and ground, while the presence of organic insulating material between various components of the primary conductor does not constitute a real disadvantage. The invention aims to provide a current transformer in which the disposition of the component parts allows simplified insulation, improved accessibility and consequently advantageous implementation.